Merge #20884: script: Improve robustness of bitcoind.service on startup

9d02654677 doc: Fix systemd spelling and link to doc/init.md (Hennadii Stepanov)
601778c310 script: Add Documentation key to bitcoind.service (Hennadii Stepanov)
d9392b724c script: Improve robustness of bitcoind.service on startup (Hennadii Stepanov)

Pull request description:

  If network interfaces are not properly up the following happens:
  ```
  ...
  2021-01-08T10:17:11Z scheduler thread start
  2021-01-08T10:17:11Z libevent: getaddrinfo: address family for nodename not supported
  2021-01-08T10:17:11Z Binding RPC on address 127.0.0.1 port 8332 failed.
  2021-01-08T10:17:11Z HTTP: creating work queue of depth 16
  2021-01-08T10:17:11Z Using random cookie authentication.
  2021-01-08T10:17:11Z Generated RPC authentication cookie /var/lib/bitcoind/.cookie
  2021-01-08T10:17:11Z HTTP: starting 2 worker threads
  2021-01-08T10:17:11Z init message: Loading banlist...
  2021-01-08T10:17:11Z SetNetworkActive: true
  2021-01-08T10:17:11Z Error: Cannot resolve -externalip address: <EDITED>
  2021-01-08T10:17:11Z Shutdown: In progress...
  2021-01-08T10:17:11Z scheduler thread exit
  2021-01-08T10:17:11Z Shutdown: done
  ```

  This PR improves robustness on startup in such cases in documented way:
  https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/

  Also minor doc improvements are added.

ACKs for top commit:
  Sjors:
    ACK 9d02654
  practicalswift:
    ACK 9d026546778629472574b26fa73338efc63d02da: patch looks correct
  darosior:
    ACK 9d02654677 -- been using the first patch too

Tree-SHA512: 38294f5682c09e6ea9008de7d7459098c920cf1b98ad8ef8a5d2ca01f2f781c0fec5591dc40ef36eeb19d94991b0c7fb7cb38c4e716bc7219875c9bcd0a55e1b

contrib/init/README.md

@@ -1,6 +1,6 @@
 Sample configuration files for:
 ```
-SystemD: bitcoind.service
+systemd: bitcoind.service
 Upstart: bitcoind.conf
 OpenRC:  bitcoind.openrc
          bitcoind.openrcconf
@@ -9,4 +9,4 @@ macOS:   org.bitcoin.bitcoind.plist
 ```
 have been made available to assist packagers in creating node packages here.
 
-See doc/init.md for more information.
+See [doc/init.md](../../doc/init.md) for more information.

contrib/init/bitcoind.service

@@ -11,7 +11,11 @@
 
 [Unit]
 Description=Bitcoin daemon
-After=network.target
+Documentation=https://github.com/bitcoin/bitcoin/blob/master/doc/init.md
+
+# https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/
+After=network-online.target
+Wants=network-online.target
 
 [Service]
 ExecStart=/usr/bin/bitcoind -daemon \